The present invention relates to medical diagnostic ultrasonic imaging, and in particular to improved methods used in connection with the alignment of two or more partially or entirely overlapping images.
U.S. Pat. No. 6,014,473, filed Aug. 22, 1997, assigned to the assignee of the present invention and hereby incorporated by reference in its entirety, describes systems for acquiring, aligning and registering multiple medical diagnostic ultrasound images. Such alignment is used both to determine the motion between two selected images as well as to provide the information needed to composite an extended image from two or more selected ultrasound images. That is, two coplanar tracking images can be aligned and in this way the relative motion of the transducer between the times of the two tracking images can be obtained. Similarly, two or more substantially coplanar diagnostic images can be aligned and then composited to form an extended field of view.
The alignment method described in U.S. Pat. No. 6,014,473 aligns two images in translation and rotation as follows. An array of rotation axes is defined. For each of these axes a test block from the respective parent image is rotated about it by a few angular steps (e.g. −4 to +4 degrees). For each rotation, the test block is then translated in each of X and Y until an optimal match is found using the minimum sum of absolute differences (MSAD) technique. The search is thus:
For each test axis location, loop over:For each rotation amount, loop over:For each translation in X, loop over:For each translation in Y,Calculate SAD,Is SAD<MSAD?Y: Update MSAD and continue searchN: Continue search
This alignment method is effective, but in some applications it requires substantial calculation time to find the best alignment.
The present invention is directed in part to an improved alignment method that is particularly efficient and quick to execute.